With the recent increase in use of portable telephones, there has been an increasing demand for new services such as high-speed data communication for transferring images and moving pictures, in addition to conventional one-to-one audio communication service. A PoC service in which the 3GPP (the third Generation Partnership Project) produces a fundamental specification is also expected to increase as one such new service (see the Nonpatent Document 1).
FIG. 1 shows one example of a mobile communication system that provides a PoC service. In the present example, a portable telephone having a PTT button as shown in enlargement at the right upper side of the drawing is used for the terminal matching the PoC. When the PoC-matched terminal is used, a user can talk to plural persons within the same group by depressing the PTT button, and can listen to these persons talk by releasing the finger from the PTT button.
In FIG. 1, a telecommunications business provider that provides the PoC service installs a PoC server 1 within a provider network including an IP packet communication link radio network. When a user dials a number (special number) of a group in which the user wants to participate, using a PoC-matched terminal A or B present in a radio area 3 or 4, the corresponding PoC-matched terminal A or B first establishes a communication link with the IP packet communication link radio network. Next, the corresponding PoC-matched terminal A or B establishes a communication link with an SIP (Session Initiation Protocol) control unit 11 of the PoC server 1 using this network based on the SIP protocol.
After a communication link is established, when a user A of the PoC-matched terminal A depresses the PTT button and talks, voice is converted into an RTP (Real-time Transport Protocol) packet, and is transmitted to the PoC server 1. The PoC server 1 multicasts the received RTP packet to PoC-matched terminals B and C that belong to the same group as that of the PoC-matched terminal A, via an RTP bridge unit 12. In the present example, the PoC-matched terminal C is present in an out-zone in which radio communication is not possible. Therefore, the PoC-matched terminal C cannot receive the RTP packet.
The above PoC server and the PoC service system can be similarly applied to an audio teleconference system and a videoconference system via a network. In this case, when plural conference terminals access a conference server within the IP network and receive an RTP packet containing audio information and image information transmitted from a conference terminal having the conference server, the RTP packet is multicast to conference terminals of other users participating in the same conference.
Nonpatent Document 1: 3GPP TR 23.070 v.0.4.0 (2004-05)